It has been proposed to cure such a polyurethane adhesive by moisture from the atmosphere, with a small amount of triethanolamine also added to the composition to act as an auxiliary curing agent. In order to achieve a reasonable cure speed, however, it was found that a relatively high temperature was required to cure the adhesive. Temperatures of the order of about 200.degree. C. for about 3 minutes have been found necessary, with consequent requirements for significant amounts of energy for heating the adhesive, and a significant time for cooling the product, and also risk of heat damage to sensitive parts.
Attempts to improve the cure speed by use of other amine curatives in addition to atmospheric moisture led us to improved cure properties. However, presence of free amine in the adhesive composition is regarded as a potential health hazard, and influences solution stability and cost of the adhesive.
For various reasons, it is often preferred to employ if possible a curable single part adhesive composition, in which all necessary ingredients for cure of the composition are present. In the case of polyurethane adhesives, this necessitates precautions to control untimely reaction between the isocyanate component and the curative, and it is necessary to achieve a satisfactory combination of storage stability and speed of cure when required.
It is known to employ single part compositions comprising a blocked isocyanate and a curative, see for example UK No. 1529319 and UK No. 1085454. Various blocking agents are known, for example phenols, lactams and ketoximes. It is known that phenol blocked isocyanates can react with hydrogen atoms of amino compounds to release the phenol and yield a polyurea which may have valuable properties in respect of mechanical and chemical resistance. An important result of such a reaction is release of the phenol which is regarded as unacceptably obnoxious e.g. in odour, if released to the atmosphere. By use of ketoxime blocked isocyanates one may realise various advantages stemming from inter alia the ability of the blocked isocyanate to be unblocked at a convenient temperature, and the comparatively unobjectionable nature of the released blocking agent which may remain in the composition when cured.
Amine cured adhesive compositions provide many of the properties required of flocking adhesives. However, amine curatives which we have found most beneficial in leading to acceptable heat cure conditions and mechanical and physical properties are also regarded as a potential health hazard when present as such in an adhesive composition subjected to heating.
We have now found that an improved one-part storage stable composition may comprise a blocked polyurethane prepolymer formed from specific amounts of selected raw materials.